1. Field of the Invention
The present invention relates to a method and device for optical fiber transmission.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a 1.55 xcexcm band can be obtained.
Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
When two waves having different frequencies are superimposed on each other, beat is generated at a frequency corresponding to the difference between these frequencies of the two waves. For example, when two lightwaves having different frequencies propagate in an optical fiber, beat having a difference frequency is generated to oscillate the material state of the optical fiber. As a result, this oscillation has an effect on the propagated light to cause scattering of a part of the propagated light. The scattered light also propagates in the optical fiber. The frequency of the scattered light is shifted from the frequency of the original propagated light by the frequency of the oscillation of the fiber material state. This property is the same as that the propagated light is modulated to generate modulated sidebands.
The frequency of the oscillation of the fiber material state is the same as the frequency of the beat, or the difference frequency in WDM signal light. Therefore, the frequency of the scattered light is equal to (the frequency of the propagated light)xc2x1(the difference frequency). Thus, the two waves at different frequencies causing the beat and the scattered wave from the propagated light produce a new, or fourth wave having another frequency (Four-Wave Mixing: FWM).
The condition for generation of the four-wave mixing is determined mainly by the powers of optical signals, the frequency (wavelength) spacings of optical signals, and the dispersion of the optical fiber. In the case that the powers are large, the frequency spacings are narrow, and the fiber dispersion is small, the four-wave mixing is easily generated.
When the frequencies of optical signals in WDM signal light are equally spaced, the frequencies of light generated by four-wave mixing are superimposed on the frequency of the transmitted optical signals. In this case, the light generated by four-wave mixing acts as noise to the optical signals, thus degrading the quality of the optical signals. The degree of this degradation depends on the optical signal power per channel, the number of WDM channels, the frequency spacing, the transmission distance, the kind of the optical fiber, and the like.
In an existing system, the generation of four-wave mixing is suppressed by lowering optical output power or using an optical fiber having a large dispersion. However, lowering the optical output power causes a corresponding reduction in system gain, resulting in a short transmission distance. Further, using an optical fiber having a large dispersion causes a difficulty of long-haul transmission due to the effect of dispersion.
Measures for suppressing the effect of four-wave mixing on the transmitted optical signals as allowing the generation of four-wave mixing include arranging the frequencies of the optical signals at unequal spacings or shifting the wavelengths of the optical signals to wavelengths longer than the zero-dispersion wavelength of the optical fiber to provide dispersion. However, when the frequency spacings are made unequal, the number of channels that can be provided in a limited band is reduced, causing a limitation to expansion of the transmission capacity. Further, when the wavelengths of optical signals are shifted to longer wavelengths with respect to the zero-dispersion wavelength of the fiber to provide dispersion, the transmission distance is reduced by the effect of dispersion.
It is therefore an object of the present invention to provide a method and device for optical fiber transmission which can suppress the effect of four-wave mixing.
In accordance with an aspect of the present invention, there is provided a method comprising the steps of wavelength division multiplexing a plurality of optical signals having different wavelengths to obtain resultant WDM signal light; transmitting the WDM signal light by an optical fiber transmission line; supplying reference light having a predetermined wavelength to the optical fiber transmission line; generating four-wave mixing in the optical fiber transmission line by the interaction of the WDM signal light and the reference light; detecting the power of light generated as the result of the four-wave mixing; and controlling the power of the WDM signal light to be supplied to the optical fiber transmission line according to the power detected.
In accordance with another aspect of the present invention, there is provided a device comprising an optical fiber transmission line; means for supplying WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, to the optical fiber transmission line; a light source for supplying reference light having a predetermined wavelength to the optical fiber transmission line; a power detector for detecting the power of light generated as the result of four-wave mixing generated in the optical fiber transmission line by the interaction of the WDM signal light and the reference light; and means for controlling the power of the WDM signal light to be supplied to the optical fiber transmission line according to the power detected.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.